Agricultural growers and researchers are constantly seeking means to economically improve root crop yields. One increasingly popular method involves the use of "beneficial" microorganisms which stimulate plant growth and improve crop yield.
A variety of mechanisms have been identified as being responsible for such plant growth promoting activity. For example, certain microorganisms directly enhance plant growth by producing growth hormones, which stimulate root crop growth directly, or by assisting in the uptake of nutrients by the crops. Alternatively, the beneficial microorganisms may promote growth indirectly by inhibiting the soil population of harmful microorganisms which limit root crop growth. Exemplary of such antagonistic microorganisms are those which produce antibiotics that kill the harmful micro-organisms and those which scavenge scarce soil nutrients in preference to the harmful microorganisms. The practice of inoculating the seeds or the soil with such beneficial microorganisms is generally referred to as "bacterization" regardless of the particular mechanism involved.
The efficacy of such beneficial microorganisms, referred to hereinafter as plant growth promoting rhizobacteria (PGPR), in promoting plant growth is highly dependent on their ability to successfully colonize the plant roots and the soil surrounding the roots (termed the "rhizosphere"). The ability of particular PGPR to colonize soil and promote plant growth is generally variable and dependent on the nature of the soil and on a number of environmental and biotic factors. It would be desirable to provide PGPR having improved capability to proliferate in different soil types which would, in turn, enhance the ability of the PGPR to promote root crop growth.